Design of Epitaxially Strained Ag Film for Durable Ag-Based Contact to p-Type GaN

We demonstrated a novel contact structure of a silver (Ag) reflector that had an enhanced thermal stability at an elevated temperature. The Ag film was epitaxially grown on GaN by the insertion of an ultrathin Ni contact layer (similar to 10 angstrom). The Ag film in the Ni/Ag structure, which was tensilely strained due to the expansion of the lattice constant by oxidation of Ni (3.532 angstrom) to NiO (4.176 angstrom), could compensate for the thermal compressive stress of 0.83 GPa that developed at the interface of Ag with GaN at 500 degrees C. The tensile stress, as determined by high resolution X-ray diffraction, was 0.24 GPa for a single period of the Ni/Ag contact, but it increased to 0.65 GPa for triple periods of the Ni/Ag contact, resulting in further suppression of Ag agglomeration and improved long-term thermal stability.